High intensity arc lamp

ABSTRACT

An improvement to a short arc gas discharge lamp is described. The anode is placed in the base of the lamp and the reflector is supported with reference to the cathode adjacent to the window. This allows more massive and simpler structures to be used for heat dissipation. The window edge is supported in compression. The improved structure includes a capacitive reactance which prevents sparking between the reflector and anode during start up.

United States Patent 119 I McRae et al.

[ HIGH INTENSITY ARC LAMP Inventors: Russell C. McRae,

Cupertino;

William R. Stuart, San Carlos, both of Calif.

Assignee:

Filed: Dec. 26, 1972 Appl. No.: 317,906

Varian Associates, Palo Alto, Calif.

Related US. Application Data Continuation of Ser. No. 109,537, Jan. 25;1971,

abandoned.

us. c1 313/113, 313/44, 313/220 Int. Cl. Hlj /16 Field of Search313/113, 220, 44

References Cited UNITED STATES PATENTS Marrison 313/1 13 Apr. 30, 19743,495,118 2 1970 Richter ..3l3/216 3,549,934 12/1970 Peacher 313/220Primary Examiner-John Kominski Attorney, Ager t, or Firm-Stanley Z.Cole; Lee F. Herbert; John J. Morrissey ABSTRACT 15 Claims, 7DrawingFigures EH 2 I l Ii 1 HIGH INTENSITY ARC LAMP This is a continuation ofapplication Ser. No. 109,537 filed Jan. 25, 1971, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a gaseousdischarge device and particularly to a novel improvement in a highintensity short arc lamp structure.

In optical projection systems involving the generation and preciselycontrolled radiation of long pulses of non-coherent'light, such as inspectroscopy, microscopy, and solar simulation, in addition to the moreconventional projection systems, there is a need for a light sourcecapable of producing the highest possible light flux density, that is,the greatest total amount of light from the least possible volume. Theideal would be a point source of light with unlimited light output.

Of the electrical devices for the generation of noncoherent light inpulses of substantial length, gas discharge devices offer thepossibility of generating the greatest total quantity of light from theleast possible volume (i.e., light flux density). The light flux densitywhich can be produced by incandescent or luminescent devices is limitedby the amount of power that can be concentrated in the solid materialswhich serve as the light emitters before a change of state occurs insuch material, whereas in a gas discharge device no such change of statecan occur in the light emitting medium regardless of the concentrationof power.

The amount of power which can be concentrated in a gas discharge maybemaximized by decreasing the spacingbetween the electrodes of the deviceand increasing the pressure of the gaseous medium, the voltage at whichthe discharge operates, and the current carried by the are. It has beenfound that for any given voltage and current the greatest light fluxdensity will be obtained when the electrode spacing and gas pressure areadjusted to produce an arc discharge which is roughly spherical (thatis, the length of the arc is approximately equal to its transversedimensions). In this mode of operation the electrode spacing is lessthan two centimeters and usually less than one centimeter. Arc dischargedevices designed to operate in this mode are called short arc devices todistinguish them from other forms of arc discharge such as medium arcand long arc devices which may produce larger total quantities of lightbut at much lower light flux density.

This invention is an improvement to the short are lamp disclosed in U.S.Pat. No. 3,502,929 which issued Mar. 24, 1970, to John F. Richter. Thisprior art lamp comprises a sealed envelope, a portion of which isceramic. The envelope houses a cathode and an anode which are spacedapart a distance less than 2 centimeters to define a short arc gaptherebetween. The envelope also houses an ionizable gas typically underapproximately standard atmospheres of pressure. A sapphire window formsone end of the envelope and a reflector is attached to the other end ofthe envelope, either as part of the envelope or separate therefrom.

In the usual embodiment of this prior art lamp, the anode is suspendedon the axis of the lamp adjacent to the window. As much as 70 percent ofthe energy of the discharge is converted into heat at the anode. Thisheat must be dissipated through the structure supporting the anode andthen through flanges thermally connected to the support structure. Thesupport structure is thin because light must pass by it. The flanges arethin to allow rapid dissipation of the heat in the critical area of theseal at the window. While highly effective at power consumptions up toapproximately watts, at powers above this level heat cannot be properlydissipated and the seals break down. These seals themselves must be thinto allow for proper expansion as they absorb heat.

Reversing the positions of the cathode and anode so that the cathode isadjacent to the window places the anode in the-base of the lamp wheremore massive structures can be used to dissipate heat. This allows forbetter dissipation of the heat produced but this effect is limited bythe requirement that the reflector mounted in the base be thermallyinsulated from the anode. However, placing the cathode adjacent to thewindow with the anode adjacent to the base gives rise to a new problemwhich offsets the advantages of higher heat dissipation. The point ofhighest intensity in the arc discharge depends on the position of thecathode, not the anode. This point must be at the focal point of thereflector to give the greatest light flux density. Small errors inposition greatly reduce light flux density. When the cathode is in thebase, the positioning of the cathode and therefore the positioning ofthe point of highest intensity relative to the focal point of thereflector is fairly easy because the reflector and cathode are in thesame. assembly. However, when the cathode is adjacent to the window, anyerror in the positioning of the point of highest intensity is difficultto detect. For this reason, the usual embodiment of the prior art lamphas the anode adjacent to the window, thus insuring the most light fluxdensity.

SUMMARY OF THE INVENTION Briefly described, the present invention is animprovement to high intensity short arc lamps. The lamp is a sealedenvelope comprising a base and anode thermally connected to the base, awindow opposite the base, a cathode insulatively supported adjacent tothe window, and a reflector supported adjacent the window so as to bereferenced to the cathode. The window is supported so as to be undercompression at the edge of the window.

Accordingly, it is an object of the present invention to provide animproved high intensity are lamp which can operate at higher power thanthe prior art.

Another object of the-present invention is to provide a lamp with higheracoustic resonance frequency so that the lamp can be modulated at higherfrequencies.

Another object of the present invention is to provide a lamp whichallows the use of simpler seals by placing these seals in areas of lessstress.

Another object of the present invention is to provide a lamp whichallows for stronger and more convenient front mounting of the lamp.

Another object of the present invention is to provide a lamp which has alonger external insulation between the cathode and anode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view ofone embodiment of the prior art lamp disclosed in US. Pat. No.3,502,929. FIG. 1A is a frontal view of the lamp in FIG. 1 along theline 1A1A in the direction of the arrows.

FIG. 2 is a cross-sectional view of one embodiment of the high intensityshort are lamp structure of the present invention. FIG. 2A is a frontview of the lamp in FIG. 2 along the line 2A2A in the direction of thearrows. FIG. 2B is a rear view of the lamp in FIG. 2 along the line 2B2Bin the direction of the arrows.

FIG. 3 is a top view of one type of radiator which may be attached tothe base of the lamp of this invention. FIG. 3A is a side view of thisradiator along the line 3A-3A in the direction of the arrows.

DESCRIPTION OF PRIOR ART FIGS. 1 and 1A illustrate an embodiment of theprior art lamp. One end of the ceramic cylinder section 40, which ismade of polycrystalline alumina, is brazed to a ductile metallic(copper, for example) ring 42, which in turn is brazed to a metallic(Kovar or stainless steel, for example) member 44 of the lamp envelope.The metallic member 44 may be spherical, ellipsoidal or parabolic. Theductile metallic ring serves as a stress relieving portion of theenvelope. The inner surface of member 44 serves as integral reflector46. The other end of ceramic member 40 is brazed to a ductile metallicring 48, which in turn is brazed to one side of a rigid metallicterminal ring 50. The terminal ring is then brazed to another ductilemetallic ring 52 which in turn is brazed to the flange of a tubularrigid metallic window support 54. As in the case of ring 42, the ductilemetallic rings 48 and 52 serve to relieve stresses. The periphery of adisc-shaped window 56, which may be a sapphire, for example, is slightlyrecessed within and brazed to window support 54.

A rod-shaped metallic anode 58 (tungsten, for example) is supportedalong the axis of tubular ceramic member 40 and window 56 by threetriangular, metallic supports 60 which may be of molybdenum, forexample. Each support has a notch into which terminal ring 50 is brazed.Supports 60 provide electrically conductive paths between anode 58 andterminal ring 50. Each of the metallic supports 60 is bent into theshape of a spiral for stress release during high temperature states.

DESCRIPTION OF PREFERRED EMBODIMENT There is illustrated in FIGS. 2, 2Aand 28 a high intensity short are lamp having a base and a ceramiccylinder section 11 of, for example, polycrystalline alumina. As will bemore thoroughly described below in the discussion of heat dissipation,the base may be constructed of steel rather than more expensive materialof high thermal conductivity. The ceramic cylinder is brazed to a ring12 having a coefficient of thermal expansion approximating that of theceramic. Such a material is an alloy of iron, nickel and cobalt soldunder the trademark Kovar. The base is recessed at 15 to form a standingedge 16. A tungsten inert gas weld is made between this standing edgeand the ring 12 at point 17. Spaces 13 and 14 are left to allowexpansion of the base. The structure and purpose of raised portion 76are discussed below.

The lamp envelope includes a disc-shaped window 18, of for example,sapphire. This window is brazed to a sealing ring 19 of, for example,Kovar, which in turn is brazed to a support member 21 which may be ofsteel. The sealing ring 19 may be very thin because it is under verylittle stress. The J-shaped cross section is convenient because itallows for an expansion of the window in a radial direction. The supportmember 21 is constructed so that a portion 25 overlaps the front of thewindow. A ductile metallic ring 20 of, for example, copper, is placedbetween the window and the overhang 25 in unsealed contact with both. Aswill be discussed more thoroughly below, this overhang 25 serves to keepthe window in compression while the ring 20 bears some of the stress.The support member 21 has a standing edge 23 which is tungsten inert gaswelded at point 24 to a sealing ring 22 of, for example, Kovar.

Because the drawing in FIG. 2 is a cross section of many generallyring-shaped members, several lines appear parallel to the window 18which may lead one to believe that some member other than the windowfills the aperture of the lamp. Each of these lines results from one ofthe generally ring-shaped members. Line 101 is the edge of the topsurface of the ring-shaped standing edge 23. Line 102 is the edge of theringshaped surface of overhang 25 of member 21. Line 103 is the edge ofthe top surface of ring 20. Finally, line 104 is the edge of thering-shaped surface of the bottom of sealing ring 19.

The three thin rectangular supports 31 for the rodshaped metalliccathode 32 are brazed to the bottom of support ring 30. These supportsare of molybdenum, for example. The ring 30 is brazed to the sealingring 22 at a point such that the supports 31 rest on the top of theceramic ring 11. The supports 31, ring 30 and ring 22 provide anelectrically conductive path to the cathode. The ring 30 has a standingedge portion 35. The reflector 33 contains slots 34 through which thecathode supports 31 pass when the top of the reflector is positioned atthe standing edge 35 of ring 30. The standing edge 35 is joined toreflector 33 with a tungsten inert gas weld. The ring 30 must besufficiently rigid to support the cathode and the reflector. A ring ofKovar has this property. The reflector is shown as a paraboloid but itmay be, for example, a spherical or ellipsoidal reflector.

A rod-shaped metallic anode penetrates through the base 10. To maximizeheat transfer from the anode, the portion which extends into the lampshould be as short as possible when compared with the thickness of thebase. Specifically, the ratio of the portion of the anode within thelamp to the maximum thickness of the base in the area below hole 36should be less than 0.6. This anode is preferably of tungsten while thecathode 32 is preferably of thoriated tungsten. The anode and cathodeare positioned on the axisof the lamp, with the axis passing through ahole 36 in the bottom of the reflector. The anode and cathode are spacedapart by less than two centimeters, preferably less than one centimeter.The cathode is positioned so that the point of greatest light intensityin the arc discharge is at the focal point of the reflector 33.

After assembly the lamp is exhausted through tube 71 and then gas filledwith, for example, xenon to a pressure approximately 25 atmospheres. Thetube is then sealed at the pinch-off 72.

While a lamp of this type will operate at very low voltages, such asvolts, once in operation, high voltages must be applied across the gapto start the lamp. Voltages such as 20,000 volts are required for thispurpose. These voltages are tyically applied by a high frequency R.F.source. Because the reflector is in electrical contact with the supportfor the cathode, one path for the RF. current is along the reflector andacross the gap at hole 36 to the anode. To prevent this arcing acrossthe gap between reflector and anode, the base has a raised portion 76whose inner surface is generally parallel to the outer surface ofreflector 33. The outer surface of the raised portion 76 is parallel tothe ceramic cylinder 11 and separated thereform by the gap 14. The gappermits expansion of the base. A capacitive reactance exists betweenthis raised portion 76 and the reflector 33. This capacitance, inconjunction with the other electrical properties of the lamp structureconsidered as a passive network, especially the inductanceof the cathodeand cathode support structure, makes possible a condition duringstarting of the lamp in which the main gap between the cathode and anodewill break down as desired, but the gap between the edge of thereflector and the anode will not break down.

As mentioned as much as 70 percent of the energy created in thedischarge is converted to heat at the anode of a short are lamp. Withthe anode of the prior art lamp positioned adjacent to the window, asshown in FIG. 1, this heat had to be dissipated through relatively thinsupports 60 and ring 50. This limited the operation of this'embodimentof the prior art to a power of approximately I50 watts. Placing theanode in the base of the prior art lamp does allow for better heatdissipation but because the reflector support 44 must in turn besupported in the base of the lamp and because the reflector cannot bepermitted to exceed relatively low temperatures, even the relativelymore massive heat transfer members which would be possible in such anembodiment would still dissipate heat less easily than the base of thecurrent invention. Such a prior art lamp could be operated at powers upto perhaps 500 watts. However, it should be remembered that placing theanode in the base of the prior art lamp creates difficulty inpositioning of the cathode relative to the focal point of the reflector.This difflculty serves to offset any advantage gained in heatdissipation. In the present invention the reflector is not attached tothe base. This allows for a simple, more massive base with a relativelyshort portion of the anode within the envelope. Heat is transferred veryquickly to the outer surface of the base so that the lamp may beoperated in the range of 500 to l,000 watts even with a base made ofsteel, a relatively poor thermal conductor when compared with othermetals which are available. The lamp will operate at even higher powersif one of these other high thermally conductive metals is used for thebase.

The relatively massive and simple form of the base makes it possible toattach radiators of relatively simple construction. Such a radiator isshown in FIGS. 3 and 3A. The radiator consists of a cylinder 81, a ring82 spaced radially from the cylinder, and a serpentine fin placedbetween and attached to the cylinder and the ring. The top of both thefin and the ring are flush with the top of the cylinder. However, asshown in FIG. 3A, the cylinder extends longitudinally below the finwhile the fin extends longitudinally below the ring. Three holes 75 arethreadedly bored into the base 10 of the lamp. Bolts (not shown) arethen inserted through holes 76 in the cylinder and into the holes 75 inthe base. The simple construction of the base allows the outer surface79 of the base 10 of the lamp to be in thermal contact with the entireupper surface of the radiator. A hole 77 in the center of the cylinder81 accommodates the anode and the pinch seal 72. Such a radiator is ableto dissipate all of the heat generated in a lamp operated at powers inthe range 500 to 1,000 watts. Other radiators of more efliciency can beused to dissipate the heat in higher power lamps.

In the prior art lamp, shown in FIG. 1, the window support 54 had to berelatively thin to allow for expansion and to properly connect with theother thin support members. Because the window is under high pressureduring the operation of the lamp, there is severe stress on the support54 and its seal with the window. In applicants invention heatdissipation in the area of the window is not critical, and the windowsupport structure of FIG. 2 allows the window to be placed incompression. This means that a thin seal 19 is possible with thiswindow, with this sea] under little or no stress.

The member 21 in FIG. 2 is relatively massive when compared with thethin flanges of the prior art. This allows for more convenient andstronger front mounting. In the embodiment shown in FIG. 2, holes arethreadedly drilled into the member 21 to receive bolts (not shown) fromany convenient mounting device.

In some applications using a short are lamp, it is desirable to modulatethe current across the arc gap, thus modulating the light produced bythe lamp. If the modulation frequency is at or near an acousticresonance frequency of the lamp, the gas molecules in the lamp willoscillate. This causes the pressure at the arc gap to vary from nearzero to maximums far greater than the normal operating pressure of thelamp. At these maximums the current in the arc gap. can no longer bemaintained and the lamp will be extinguished. It is desirable to havethe lowest of the lamps acoustic resonance frequencies as high aspossible so that it will exceed any modulating frequency which might beused. This lowest acoustic resonance frequency increases as the volumeof gas decreases.

Both the present invention and the prior art lamp consist-of a pair ofcoupled cavities. The lowest acoustic resonance frequency is a complexfunction of the resonance frequencies of each of these coupled cavities.However, the major contribution comes from the cavity in which the arcgap is located. In the present invention the distance from the bottom ofthe window 18 to the top of the reflector 33 is much less than in theprior art lamp. This is because the reflector is supported adjacent tothe window and therefore adjacent to the cathode, requiring noinsulation from the cathode in the area of the window. This means thatthe window can be quite close to the reflector. ln the preferredembodiment the distance from the window to the reflector is less thanthe thickness of the window. Thus for the same aperture size and shapeof reflector, the volume of gas in the chamber which includes the arc issmaller in the present invention than in the prior art.

With the reflector attached adjacent to the window, the ceramicinsulator 11 is much longer than in the prior art lamp. This means thathigher starting voltages can be used, or alternatively, that there isless likelihood of arc-over at low external pressures, e.g. at highaltitude.

What is claimed is:

1. An arc lamp comprising a sealed envelope having a base, an opticalwindow disposed in said envelope opposite said base, said envelopeincluding an annular insulating member between said base and saidwindow, said envelope containing an ionizable gas at a pressure higherthan atmospheric pressure, a first electrode within said envelope, areflector within said envelope, support means supporting both said firstelectrode and said reflector on said insulating member adjacent thewindow end of said insulating member, a second electrode mounted withinsaid envelope adjacent said base, and means supporting said secondelectrode from the other end of said insulating member.

2. The arc lamp of claim 1 wherein said envelope includes an annularwall portion adjacent said window end of said insulating member, andwherein said support means for said first electrode and reflectorincludes an annular ring the outer portion of which is attached to saidannular wall portion of said envelope and the inner portion of which isattached to said reflector and to said first electrode.

3. The are lamp of claim 1 wherein said reflector is metallic and iselectrically connected to said first electrode.

4. The arc lamp of claim 1 wherein said reflector is configured as asurface of revolution and said first electrode is cylindrical, the axisof said first electrode coinciding with an axis of said reflector.

5. The arc lamp of claim 1 wherein said reflector is a metallic surfaceof revolution, said first electrode is a cathode and said secondelectrode is an anode, said support means includes an annular ring uponwhich said reflector is mounted and to which said cathode is attached byat least one electrically conducting support member which supports saidcathode generally coaxially with an axis of said reflector, said supportmember being elongated in a direction normal to said axis, saidreflector and said cathode being electrically interconnected.

6. An arc lamp comprising a hermetically sealed envelope containing afirst electrode and a second electrode, said envelope comprising aninsulating member electrically separating said electrodes to form an arcgap, an optical window at one end of said lamp, said first electrodebeing positioned adjacent the window end and said second electrode beingpositioned adjacent the opposite end of said envelope, a reflectorcomprising a surface of revolution positioned to reflect light from saidare gap through said window, said reflector having a large diametercross section at one end, said large end of said reflector beingpositioned closely adjacent said window, and said insulating memberbeing positioned to extend from said reflector toward said secondelectrode.

7. The are lamp of claim 6 wherein the distance from the surface of saidwindow nearest said arc gap to said reflector is less than the thicknessof said window.

8. The arc lamp of claim 6 wherein said opposite end of said lampcomprises an electrically conductive base electrically connected to saidsecond electrode, and said first electrode is electrically connected tosaid reflector.

9. An arc lamp comprising a hermetically sealed envelope containing anionizable gas at a pressure higher than atmospheric pressure, saidenvelope having an optical window portion, sealing means forhermetically sealing said window to the non-window portion of saidenvelope, said sealing means comprising a flexible member to accommodatelateral expansion of said window, and abutment means overlapping theouter surface of said window for compressively restraining said windowfrom movement in response to the outwardly directed pressure of said gaswithout stress by said outwardly directed pressure upon said sealingmeans.

10. The are lamp of claim 9 wherein said abutment means is annular witha portion which extends outside said envelope to circumferentiallyoverlap the outer surface of said window so as to form an abutment lipthereagainst.

11. The are lamp of claim 10 wherein said sealing means comprises a thindeformable sealing member one end of which has a first sealing portionconnected to said window and the other end of which has a second sealingportion connected to said annular abutment means, said sealing memberhaving a flexible portion between said first and second sealing portionswhich is deformable to accommodate the difference in coefficient ofexpansion of said window and said abutment means.

12. An arc lamp comprising a sealed envelope having a base, an opticalwindow disposed in said envelope opposite said base, said envelopecontaining an ionizable gas at a pressure higher than atmosphericpressure, a cathode and a metallic optical reflector mounted within saidenvelope upon support means disposed adjacent said window, and acylindrical anode mounted within said envelope upon said base andextending toward said cathode, said base comprising a heat conductivematerial in surface contact with said anode for at least sixty percentof the length of said anode within said envelope.

13. The arc lamp of claim 12 wherein said heat conductive material has arecess in its inner surface with the walls of said recess extendinginwardly beyond the inner end of said anode.

14. The are lamp of claim 13 wherein the walls of said recess projectinwardly to surround a portion of said reflector. 7

15. An arc lamp comprising a sealed envelope having a base, an opticalwindow disposed in said envelope opposite said base, said envelopecontaining an ionizable gas at a pressure higher than atmosphericpressure, a first electrode mounted adjacent said window, a reflector,support means upon which said first electrode and said reflector aremounted within said envelope adjacent said window, a second electrodemounted within said envelope adjacent said base, and a dielectric memberfor electrically insulating said second electrode from said firstelectrode.

1. An arc lamp comprising a sealed envelope having a base, an opticalwindow disposed in said envelope opposite said base, said envelopeincluding an annular insulating member between said base and saidwindow, said envelope containing an ionizable gas at a pressure higherthan atmospheric pressure, a first electrode within said envelope, areflector within said envelope, support means supporting both said firstelectrode and said reflector on said insulating member adjacent thewindow end of said insulating member, a second electrode mounted withinsaid envelope adjacent said base, and means supporting said secondelectrode from the other end of said insulating member.
 2. The arc lampof claim 1 wherein said envelope includes an annular wall portionadjacent said window end of said insulating member, and wherein saidsupport means for said first electrode and reflector includes an annularring the outer portion of which is attached to said annular wall portionof said envelope and the inner portion of which is attached to saidreflector and to said first electrode.
 3. The arc lamp of claim 1wherein said reflector is metallic and is electrically connected to saidfirst electrode.
 4. The arc lamp of claim 1 wherein said reflector isconfigured as a surface of revolution and said first electrode iscylindrical, the axis of said first electrode coinciding with an axis ofsaid reflector.
 5. The arc lamp of claim 1 wherein said reflector is ametallic surface of revolution, said first electrode is a cathode andsaid second electrode is an anode, said support means includes anannular ring upon which said reflector is mounted and to which saidcathode is attached by at least one electrically conducting supportmember which supports said cathode generally coaxially with an axis ofsaid reflector, said support member being elongated In a directionnormal to said axis, said reflector and said cathode being electricallyinterconnected.
 6. An arc lamp comprising a hermetically sealed envelopecontaining a first electrode and a second electrode, said envelopecomprising an insulating member electrically separating said electrodesto form an arc gap, an optical window at one end of said lamp, saidfirst electrode being positioned adjacent the window end and said secondelectrode being positioned adjacent the opposite end of said envelope, areflector comprising a surface of revolution positioned to reflect lightfrom said arc gap through said window, said reflector having a largediameter cross section at one end, said large end of said reflectorbeing positioned closely adjacent said window, and said insulatingmember being positioned to extend from said reflector toward said secondelectrode.
 7. The arc lamp of claim 6 wherein the distance from thesurface of said window nearest said arc gap to said reflector is lessthan the thickness of said window.
 8. The arc lamp of claim 6 whereinsaid opposite end of said lamp comprises an electrically conductive baseelectrically connected to said second electrode, and said firstelectrode is electrically connected to said reflector.
 9. An arc lampcomprising a hermetically sealed envelope containing an ionizable gas ata pressure higher than atmospheric pressure, said envelope having anoptical window portion, sealing means for hermetically sealing saidwindow to the non-window portion of said envelope, said sealing meanscomprising a flexible member to accommodate lateral expansion of saidwindow, and abutment means overlapping the outer surface of said windowfor compressively restraining said window from movement in response tothe outwardly directed pressure of said gas without stress by saidoutwardly directed pressure upon said sealing means.
 10. The arc lamp ofclaim 9 wherein said abutment means is annular with a portion whichextends outside said envelope to circumferentially overlap the outersurface of said window so as to form an abutment lip thereagainst. 11.The arc lamp of claim 10 wherein said sealing means comprises a thindeformable sealing member one end of which has a first sealing portionconnected to said window and the other end of which has a second sealingportion connected to said annular abutment means, said sealing memberhaving a flexible portion between said first and second sealing portionswhich is deformable to accommodate the difference in coefficient ofexpansion of said window and said abutment means.
 12. An arc lampcomprising a sealed envelope having a base, an optical window disposedin said envelope opposite said base, said envelope containing anionizable gas at a pressure higher than atmospheric pressure, a cathodeand a metallic optical reflector mounted within said envelope uponsupport means disposed adjacent said window, and a cylindrical anodemounted within said envelope upon said base and extending toward saidcathode, said base comprising a heat conductive material in surfacecontact with said anode for at least sixty percent of the length of saidanode within said envelope.
 13. The arc lamp of claim 12 wherein saidheat conductive material has a recess in its inner surface with thewalls of said recess extending inwardly beyond the inner end of saidanode.
 14. The arc lamp of claim 13 wherein the walls of said recessproject inwardly to surround a portion of said reflector.
 15. An arclamp comprising a sealed envelope having a base, an optical windowdisposed in said envelope opposite said base, said envelope containingan ionizable gas at a pressure higher than atmospheric pressure, a firstelectrode mounted adjacent said window, a reflector, support means uponwhich said first electrode and said reflector are mounted within saidenvelope adjacent said window, a second electrode mounted within saidenvelope adjacent said base, and a dielectric member for electricallyinsulating said sEcond electrode from said first electrode.